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Full text of "IS 14672: Method of Test for High Voltage Low Current Dry Arc Resistance of Solid Electrical Insulation"

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IS 14672 (1999) : Method of Test for High Voltage Low 
Current Dry Arc Resistance of Solid Electrical Insulation 
;etD 2 : Solid Electrical Insulating Materials and 
Insulation Systems] 




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PROTECTED BY COPYRIGHT 



IS 14672 : 1999 

( Reaffirmed 2004 ) 



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Indian Standard 

METHOD OF TEST FOR HIGH VOLTAGE 

LOW CURRENT DRY ARC RESISTANCE 

OF SOLID ELECTRICAL INSULATION 



ICS 29.035.99 



© BIS 1999 

BUREAU OF INDIAN STANDARDS 

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG 
NEW DELHI 110002 



y«ne 1999 Price Group 4 



Solid Electrical Insulating Materials Sectional Committee ETD, 02' 



FOREWORD 

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the Solid 
Electrical Insulating Materials Sectional Committee had been approved by the Electrotechnical Division Council. 

This standard described the test method used for preliminary screening of materials on the basis of their resistance 
to the action of a high voltage, low current arc close to surface of insulation which tends to form a conducting 
path or causes thermal and chemical composition and erosion. 

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, 
observed or calculated, expressing the result of a test, shall be rounded off in accordance with IS 2 : 1960 'Rules 
for rounding off numerical values (revised)'. The number of significant places retained in the rounded off value 
should be the same as that of the specified value in this standard. 



IS 14672 : 1999 



Indian Standard 

METHOD OF TEST FOR HIGH VOLTAGE 

LOW CURRENT DRY ARC RESISTANCE 

OF SOLID ELECTRICAL INSULATION 



1 SCOPE 

1.1 This test method can be used for preliminary 
screening of materials on the basis of their resistance 
to the action of a high voltage, low current arc close 
to surface of insulation which tends to form a 
conducting path or causes thermal and chemical 
decomposition and erosion. 

1.2 The test method is not applicable to materials that 
do not produce conductive paths under the action of 
an electric arc, or that melt or form fluid residues 
which prevent formation of a conductive path. 

2 GENERAL 

2.1 The test method has serious limitations as the 
conditions under which arcings occur are different. 
This test method, in general, should not be used in 
material specification and, if possible, alternate test 
methods should be used. 

2.2 The test method may not permit the classification 
of insulating materials on the strength of the relative 
arc resistance values as the arcs can be of various types, 
namely, high voltage at high current and low voltage 
at low or high currents, etc. 

2.3 The test method is recommended because of its 
convenience and short time required for testing. It is 
extremely useful for preliminary screening of identical 
materials, for studying the effect of chemical 
formulations and routine quality control testing. Since 
the tests are conducted under clean and dry laboratory 
conditions, rarely encountered in practice, the 
prediction of a material's relative performance during 
service under varied environments may not be 
accurate. Hence, a cautious approach, well supported 
by simulated service tests and field experience is 
required for drawing definite conclusions from this 
test method. 

2.4 This method uses dry and clean specimens whereas 
test methods given in IS 2824 employ wet and 
contaminated specimen surfaces. The use of both test 
methods may be useful for design and quality control 

purposes. 



3 TERMINOLOGY 

3.0 For the purpose of this standard, the following 
definitions shall apply: 

3.1 Arc Resistance 

The total elapsed time of operation of test until failure 
occurs. 

3.2 Failure 

a) The end point or failure is said to have 
occurred if a conducting path is formed across 
the dielectric and the arc disappears into the 
material. There is a significant change in the 
sound at the instance of failure. However, for 
some materials it may be difficult to precisely 
define failure. In general, at failure the en- 
tire portion of the surface between the elec- 
trodes shall carry current, that is, there should 
be no arcing in any part of the inter-electrode 
spacing. In some cases persistent scintilla- 
tions may be observed even after failure but 
this should not be considered as a part of the 
arc resistance time. 

b) Burning of the material can also obscure the 
arc. In such cases, failure should be reported 
as * failure due to burning'. 

c) In successive arc intervals the dielectric con- 
ducts the current only in the later part of the 
'on' time. The first such interval should be 
designated as failure. 

NOTES 

1 If no conducting path is formed under the action of an electric 
arc then this test method may not be applicable. 

2 The orientation of the electrodes and specimens can affect the 
test results. Normally the electrodes are placed on top of the speci- 
men. In the other configuration the specimen rests on the elec- 
trode to give a better contact. This configuration is more severe 
and also reduces the dispersion in the test results. 

4 SIGNIFICANCE 

In this test method, it is intended to simulate 
approximately the conditions which prevail in ac 
circuits operating at high voltage but at currents 
limited to a few milii-ampere. 



IS 14672 : 1999 



To distinguish materials of low arc resistance value, 
the condition in the early stages of this test method 
are mild and severity of testing is progressively 
increased. 

Generally different types of failure and have been 
observed. There are: 

a) Inorganic dielectrics become incandescent 
and thus become conducting. Upon cooling 
they regain their insulating properties; 

b) Organic compounds burst into flame with- 
out forming a visible conducting path; and 

c) Insulating materials can also fail by tracking 
and carbonization. 

While comparing the arc resistance of different 
materials due importance must be given to the stage 
at which failure occurs. For example, there is a greater 
difference between 178 s and 182 s than between 
174 s and 178 s. This is because in the former case an 
increase in severity has immediately resulted in failure. 
Hence, a judicious interpretation of test results has to 
be made. 

5 CHOICE OF ELECTRODE CONFIGURATION 

For materials with poor to moderate arc resistance (up 
to 180 s) the stainless steel electrode configuration is 
recommended. For materials with arc resistance 
greater than 1 80 s the use of tungsten rod electrodes 
is advisable because the corners of stainless steel 
electrodes undergo severe erosion under intense 
arcings. 

6 APPARATUS (see Fig. 1) 

6.1 Transformer (J ) 

A self-regulating transformer with a rated primary 
potential of 220 V, 50 Hz ac, secondary potential of 
15 kV and short circuit current of 0.060 A. 

6.2 Variable Auto Transformer ( J ) 

An auto transformer rated for 8 A or more and 
nominally adjustable to 250 V is used. 

Voltmeter V^ — An ac voltmeter readable to 1 V in 
the range of 170 to 250 V is permanently connected 
across the output of auto transformer to indicate the 
voltage supplied to the primary circuit. 

Milliammeter A — An ac milliammeter capable of 
reading from 10 to 40 mA with an error of ± 5 percent. 
The meter should be calibrated in a test circuit 
containing no arc gap. A by-pass switch is provided 
to short the ammeter when it is not used. 



ELECTRODE 
ASSEMBLY 



1,2 TO 1.5 HENRIES 
(SEE TEXT) 




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I oc I 



NOTE — Switches S^io S^^^ are aligned in the sequence of their 
closing, from bottom to top, during a test. 

Fig. 1 Arc-Resistance Test Circuit 



6.3 Current Control Resistors, R^^^ R^^, R^^j R^^ 

Four resistors are connected in series with the primary 
of transformer Tbut in parallel with each other. These 
resistors must be adjustable to permit exact setting of 
current during calibration. R^^ is always used in the 
circuit to get a current of 10 mA. Using S^^, switch S^^ 
should add R^^ in parallel to R^^ and provide current 
of 20 mA to the arc. Similarly switches S^^ and S^^ 
when switched on should provide arc currents of 
30 mA and 40 mA respectively. 



IS 14672 : 1999 



6.4 Suppressing Resistor, R^ 

R^ — Rated at 1 5 K-ohm and about 50 W. This resistor 
along with air core inductor is used to suppress 
parasitic high frequency in the arc circuit. 

6.5 Air Core Inductors 

Inductance of 1 to 1 .5 H is obtained from about 8 coils 
(of No, 30 Cotton or enamel-covered wire). A single 
coil of this inductance is not satisfactory. Each coil 
consists of 3 000 to 5 000 turns of wire wound on 
insulating or non-metallic cores of about 12.7 mm 
diameter and 16 mm inside length. 

6.6 Interrupter (i) 

This is a motor-driven device used to give the required 
cycles for the three lower steps of test by opening and 
closing of primary circuit according to the schedule 
in Table 1, with an accuracy of ±1/20 s or better. The 
interruptor can be a synchronous motor driving three 
appropriate sets of cams which actuate the contactor 
switches. 

Table 1 Sequence of 1 min Current Steps 

[Clauses 6.6, 6.10 and 9.1(d)] 



Step 


Current, mA 


Time Cycle 


Total Time, s 


1/8 10 


10 


1/4 s On, 

1 3/4 s Off 


60 


1/4 10 


10 


1/4 s On, 

3/4 s Off 


120 


1/2 10 


10 


1/4 s On, 

1/4 s Off 


180 


10 


10 


Continuous 


240 


20 


20 


Continuous 


300 


30 


30 


Continuous 


360 


40 


40 


Continuous 


420 



6.7 Timer 

An electric time interval meter operating at 220 V, 
accurate to Is. 

6.8 Indicator Lamp 

A 6W 220 V lamp with a series resistor indicates the 
interrupting cycle being used and also helps the 
operator to start the first cycle of each test in a uniform 
manner by closing 5,^g just after the lamp is 
extinguished. 

6.9 Control Switches 

Toggle switches are convenient. All switches may be 
rated for 220 V, lOA. 

6.10 Safety Interlocking Contactor (C^) 

At least two interlocking switches shall be provided 
so that raising the electrode assembly cover will 



remove the HV from the electrodes. This ensures safety 
of the operator. 

Interruptor contacts: Cj^g C^^^ C^^^ 

Normally open spring contactors, rated at 2 A or better, 
these are operated by interrupted cams, thus closing 
and opening the primary circuit and providing the 
intermittent arc cycle listed in Table 1. 

6.11 High Voltage Switch (S^) 

A single pole, single throw switch insulated for 
15 kV ac isolated from the operator by a suitable 
enclosure through which projects an insulating handle 
of sufficient length to ensure operator safety. 

6.12 Wiring 

All wiring in the arc circuit must be of ignition wire 
rated at 15 kV or higher and must be disposed so that 
any circuit component is not readily accessible when 
energized. 

6.13 Sharpening Jig for Electrodes 

Suitable sharpening jig with provision for securing 
the electrodes firmly must be used to ensure finishing 
of the pointed tips to the proper geometry {see Fig. 2), 

6.14 Stainless Steel Strip Electrodes 

Stainless Steel foil of 0.15 mm, 12.7 x 25.4 mm 
slightly bent in the middle of form an angle of 160°. 

6.15 Tungsten Rod Electrodes 

Tungsten rod of 2.4 mm, free from cracks, pits or rough 
spots is used for making electrodes. The rod is fastened 
into a square shank by brazing after leaving an exposed 
length of about 20 mm. The rod end must be ground 
to a 30° angle to the axis to achieve a flat elliptical 
face. 

6.16 Electrode Assembly 

The electrode assembly should be constructed so as to 
make the top specimen surface perfectly horizontal 
and ample air space shall be provided below the test 
area. Each electrode should independently exert a force 
of 50 ±5g on the specimen surface. A transparent 
shield must be provided to the assembly to protect the 
specimen from air drafts and allow venting of 
combustion products in case the specimen gives off 
toxic smoke during testing. The operator must have a 
clear view of the arc from a position slightly above 
the plane of the specimen. 

6.17 Stainless Steel Strip Electrode Assembly 
(Fig. 3, 4, 5 and 6) 

Two stainless steel strip electrodes are placed on the 



IS 14672 : 1999 










Fig. 2 Grinding and Polishing Block with Tungsten Rod Electrode in Place 



specimen with the corners down and spaced 

6 ± 0.05 mm apart and at an angle of 45° to a line 
joining the comers. An electrode holder or the rod 
electrode assembly can be used to hold the electrodes 
in position. 

6.18 Tungsten Rod Electrode Assembly 

The electrodes are positioned so that they lie in the 
same vertical plane and are inclined at an angle of 
35'' to the horizontal. The minor axes of the elliptical 
tip surface must be horizontal and the distance between 
them must be adjusted accurately to 6 ± 0.05 mm to 
achieve: (i) axis of the tungsten rod must be 
perpendicular to the axis of the support rod, (ii) support 
rod must be gripped in the pivot block in a position 
such that the axis of each electrode is inclined at 35'' 
when the support rods are horizontal, (iii) electrodes 
are mounted in square shanks and sharpened in a jig, 
and (iv) the spacing between electrode tips is adjusted 
with support rods in a horizontal position. 

7 SAFETY PRECAUTIONS 

The test apparatus and associated accessories must be 



properly designed and installed for safe operation. 

All metal parts that an operator can come into contact 
with must be very well grounded. 

During testing, there are chances of fire, 
explosion and rupture of the test chamber due to 
the energy released at the time of breakdown of 
the sample. The possibility of injury to the 
operator due to such accidents must be minimized 
if not totally avoided. 

8 SPECIMENS AND NUMBER OF TESTS 

a) Normally five specimens of 3 mm thickness 
measuring 25 mm x 25 mm are used. Thin 
materials can be stacked to get the desired 
thickness. 

b) For moulded parts, arc is applied to a loca- 
tion deemed to be most significant. 

c) Always clean specimens are to be used. If the 
surface is contaminated with moisture or finer 
prints, it should be cleaned with water or a 
suitable solvent. 



IS 14672 : 1999 




Fig. 3 Strip Electrodes and Holders 




Fig. 4 Strip Electrodes in Place 
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IS 14672 : 1999 




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IS 14672 : 1999 



Calibration — The test equipment must be 
periodically calibrated for correct voltage and current. 

Conditioning — The specimen must be cleaned and 
dried at 50°C for about 30 min and then conditioned 
at 55 percent RH and 27± 2°C for a period of 40 h. 

9 ARC RESISTANCE TEST 

9.1 Procedure for Measuring the Arc Resistance 

a) The specimen shall be subjected to test within 
3 min after removal from the conditioning 
chamber; 

b) The specimen is placed in the appropriate 
position with the electrodes resting on its 
face. The interelectrode distance is accurately 
adjusted. The transparent cover is then low- 
ered to cover the electrode assembly; 

c) The switch 5, is closed and 7 is adjusted to 
get the necessary voltage; and 

d) The test is commenced by closing S^, S^ and 
.S*^. As soon as the indicator lamp is off, S^ 
and 5*,^ are closed and the timer begins to 
count. At the end of every minute the arc se- 
verity is increased in the sequence shown in 
Table 1, until failure as defined occurs. The 
arc current is immediately interrupted and 
the interval timer closed by opening 5',. The 
time to failure is thus recorded. 

It is necessary to closely observe the arc during testing. 
If the arc should climb or flare irregularly, the circuit 
constants may not be correct. 

9.2 Report 

The test report shall include the following: 

a) Type and trade name of the material; 

b) Pre-conditioning and conditioning; 

c) Thickness of specimen, if not a standard 
specimen; 

d) Electrode system and orientation used; 

e) Number of tests; 

f) Median and minimum arc resistance value; 
and 

g) Remarks like burning, softening, etc. 

10 SURFACE BREAKDOWN VOLTAGE RATIO 
TEST 

10.1 Scope and Significance 

Insulating materials have a tendency to regain their 
insulating properties on cooling after the occurrence 
of failure due to surface discharges. The degree to 
which the material will recover its insulating 
properties after arcing has ceased, is of vital 



importance to the design of HV insulation system. This 
characteristic of the material determines whether an 
insulation needs replacement or not after removal of 
causes of temporary or accidental arcing. 

The conditions under which arcings occur are arbitrary 
and hence the results should not be indiscriminately 
used in situations wherein arc currents are more. A 
cautious approach is necessary for making the best 
useof the test results. 

A surface breakdown test is carried out on unaffected 
and arc affected specimen and the ratio of the two 
gives the surface breakdown voltage ratio. 

This test should not be performed when failure causes 
extensive damage to the test area. 

10.2 Procedure 

The arc resistance test is carried out in a manner 
described earlier and after failure, the specimen is 
allowed to cool to room temperature. Then switch 5,^ 
is closed and with 7 , a voltage of 200 V is applied 
across the electrodes. Switch S^ is closed for about 1 s 
and voltmeter V^ is observed for any drop in voltage. 
The voltage is constantly raised and for every 200 V, 
the switch S^ is closed for 1 s and V^ is observed. The 
voltage across the electrode at which there is a 
significant drop on closing S^ (a drop of about 40-50 
percent) is called the surface breakdown voltage. The 
experiment is also repeated on an unaffected specimen 
surface. 

The electrodes must be cleaned after every test. 

10.3 Number of Tests and Calculation 

a) At least five tests must be performed 
individually for arc-affected and unaffected 
specimens; and 

b) The average of breakdown values of un- 
affected specimen is calculated. Each value 
obtained for exposed area is divided by this 
average value to get the surface breakdown 
voltage ratio. 

10.4 Report 

The test report shall include: 

a) Type and trade name of material; 

b) Preconditioning and conditioning; 

c) Thickness of specimen, if not a standard 
specimen; 

d) Number of tests; 

e) Average and minimum of arc resistance times 
and surface breakdown voltage ratio; and 

f) Special remarks, for example, like burning, 
softening, etc. 



Bureau of Indian Standards 

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harmonious development of the- activities of standardization, marking and quality certification of goods 
and attending to connected matters in the country. 

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BIS has the copyright of all its publications. No part of these publications may be reproduced in any form 
without the prior permission in writing of BIS. This does not preclude the free use, in the course of 
implementing the standard, of necessary details, such as symbols and sizes, type or grade designations. 
Enquiries relating to copyright be addressed to the Director (Publications), BIS. 

Review of Indian Standards 

Amendments are issued to standards as the need arises on the basis of comments. Standards are also reviewed 
periodically; a standard along with amendments is reaffirmed when such review indicates that no changes are 
needed; if the review indicates that changes are needed, it is taken up for revision. Users of Indian Standards 
should ascertain that they are in possession of the latest amendments or edition by referring to the latest issue of 
'BIS Handbook' and 'Standards: Monthly Additions'. 

This Indian Standard has been developed from Doc : No. ET 02 (3525). 



Amendments Issued Since Publication 



Amend No. 



Date of Issue 



Text Affected 



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